This invention relates to an oxidative method for microcoulometric determination of sulfur in hydrocarbons.
The presence of chlorine in concentrations in excess of 1 ppm and nitrogen in excess of 0.1 ppm may interfere in the iodometric determination of sulfur in petroleum and chemical products using the iodometric ASTM Method of Test for Sulfur in Petroleum Gas by Oxidative Microcoulometry (D 3246-76) and similarly in the method for sulfur in petroleum liquid (ASTM D 3120-75) or their equivalents. In the case of nitrogen, the extent of such interference is dependent on the type of nitrogen compounds as well as the amount present.
The interference results from oxidation of nitrogen to oxides of nitrogen and from oxidation of organic and inorganic chlorides to elemental chlorine, each of which reacts with potassium iodide in the iodometric titration to liberate iodine. Sulfur is oxidized in the method to sulfur dioxide which is measured by the consumption of iodine in the titration. Therefore, the effect of the presence of nitrogen and chlorine is to cause low, or even negative results. Since chlorine, and especially nitrogen, are commonly associated with sulfur in petroleum products, petrochemicals, and chemicals, this interference constitutes a severe limitation of the above methods.
Efforts have been made to overcome such interference. U.S. Pat. No. 3,598,531 to Bremanis describes a method whereby alkali metal azides are added to a titration solvent. Apparently, the azides react preferentially with the chlorine and the harmful oxides of nitrogen before such chlorine and oxides of nitrogen can react with the potassium iodide. Although azide addition appears to improve the sensitivity and precision of the measurement for sulfur according to the ASTM method, interference still prevents accurate measurements of sulfur below 1 ppm.
It is an object of this invention to provide an improved method for determining sulfur as sulfur dioxide in petroleum and chemical products. More particularly, an object is to provide a way of overcoming nitrogen and chlorine interference in the methods of sulfur determination involving microcoulometry. Yet another object is to provide a method for determining sulfur at lower concentrations than heretofore possible with conventional microcoulometric techniques.